It's a reward mechanism. It's mere existence makes it valuable.. kind of like fiat money :) But seriously, on the micro scale, earning a single karma point fee

It's a reward mechanism. It's mere existence makes it valuable.. kind of like fiat money :) But seriously, on the micro scale, earning a single karma point feels good because it means somebody appreciates what you said. On a macro level, it lets you compete with other people, and serves the same purpose that high scores in video games do. Humans respond very well to an abstract notion of wealth, and humans are naturally competitive.